Despite the recent interest in adopting HTML5's video tag, there is still one major problem: there is no mandated standard video codec for the video tag. The two main contestants are the proprietary and patended h264, and the open and free Theora. In a comment on an LWN.net article about this problematic situation, LWN reader Trelane posted an email exchange he had with MPEG-LA, which should further cement Theora as the obvious choice.

GStreamer also runs on Windows and Mac OS X. Providing different back-ends in not needed.
As an alternative to GStreamer, Mozilla could also use Xine (compiled without patented codecs -- perfectly possible). Dirac would still be part of it, as far as I know Xine.